Field of the Invention
The present invention generally relates to an article in cast iron comprising at least a side which has a specific roughness and is provided with a vitreous coating comprising a silicone oil. The present invention also relates to a method for manufacturing a coating synthesized by sol-gel process from such an article.
Description of Related Art
By cast iron, is meant within the meaning of the present invention, a cast iron of ferrous alloy type.
By vitreous coating, is meant within the meaning of the present invention, a liquid phase precursor based solution which is converted into a solid by a set of chemical reactions (hydrolysis and condensation) at low temperature. The, thus obtained coating is an organic-inorganic hybrid coating.
By organic-inorganic coating, is meant within the meaning of the present invention, a coating whereof the network is substantially inorganic, but which comprises organic groups, particularly due to the precursors used and the curing temperature of the coating and presence of silicone oil.
In the field of sol-gel coatings, it is particularly known of those obtained from silicon based metallic alkoxydes (silanes) or aluminum based ones (aluminates).These coatings are currently witnessing an important development in the field of culinary articles, and more particularly those whereof the support is in stainless steel or aluminum. They are also used by way of non-stick coatings covering the inner cooking surface.
As regards cast iron articles, the protective and/or decorative coatings known to this day are based on enamel or vegetable oil.
As regards more particularly culinary articles in enameled cast iron, these are enameled on their inner side as well as on their outer side with matte or shiny enamels. Nevertheless, they have many drawbacks. In fact, for enameling a cast iron support, it is necessary to use a grey iron, with a chemical composition such that the mass percentage of carbon (with respect to the total weight of the support) ranges between 3.2 and 3.7%, that of silicon ranging between 2.2 and 3%, that of manganese between 0.4 and 0.7%, that of phosphorous between 0.4 and 1%, and finally that of sulfur between 0.05 and 0.1% in weight. If this is not taken into consideration, aspect defects in the enamel risk appearing after curing, for example black spots or apitting phenomenon.
However, before the enameling, an advanced mechanical treatment (strong shot-peening) is recommended in order to increase the superficial surface of the support and remove the impurities left from the molding of the article in the foundry iron, such as to improve the adherence of the enamel on the cast iron.
Furthermore, the enameling of the cast iron most of the time requires (apart from matte enamel) a double curing: a first curing at 800° C.-840° C. to degas the substrate before applying the second layer of enamel; and a second curing for the second layer of enamel at 770-800° C. The enameling method of the cast iron is lengthy and highly energy-consuming, hence costly.
Finally, due to high curing temperatures and variations in thickness of the enamel layer or in the support, the risk of rejects (particularly for alignment type defects (metal defects reproduced by the coating) is high: it is often about 20% which is higher than an order of magnitude that is generally observed with an enamel for aluminum.
Furthermore, cast iron articles coated with black-colored vegetable oil are usually attractive when new, but are known to be incompatible with healthy and practical cooking. In addition, such articles are difficult to clean and exhibit, upon usage, low hardness and low resistance to oxidization. Finally aesthetic-wise, the use of a vegetable oil to cover a cast iron support limits the range of colors to black.
However, the applicant has now developed a method making it possible to deposit a vitreous coating on a cast iron support that has dishwasher resistance and support adherence properties, which allows to prevent all the aforementioned drawbacks. Furthermore, such a method makes it possible to deposit a vitreous coating whereof the color can be selected from a range of colors as varied as for an enamel coating (without being limited to black as is the case with vegetable oil).
It is known by the skilled person to use sol-gel coatings on cast iron articles. However, these coatings are not directly in contact with the cast iron support of the article in as far as they are deposited on an intermediate enameled layer. For example, American patent application US 2011/0111239 teaches of a non-stick coating for steel or cast iron surfaces comprising a first layer of vitrified enamel obtained from a composition of enamel frit heated at a temperature of the order of 700° C., whereof a portion is made rough and whereon is deposited a layer of sol-gel coating obtained from metal alkoxydes. However, US 2011/0111239 clearly teaches that the enamel/sol-gel bi-layer coating makes it possible to avoid the drawbacks of an entirely sol-gel coating for cast iron, which would hardly be non-stick, but also rough and able to become easily corroded in the dishwasher.